The invention relates to the production of sour natural gas. More particularly, it relates to a method of producing a sour natural gas from a subterranean reservoir formation while preventing plugging of the reservoir formation due to in-situ precipitation of sulphur.
Sour natural gas is able to hold a limited amount of sulphur in solution. The amount of dissolved sulphur increases with the pressure, temperature and hydrogen sulfide content of the gas.
Elemental sulphur comes out of solution when either the pressure or temperature of the fluid drops below the saturation values. Such a change in conditions may easily occur when gas is produced in a production well.
In sour gas production operations via a conventional vertical well, significant amounts of elemental sulphur may be separated from the produced fluid. Depending on the distribution and severity of the pressure and temperature reduction throughout the flow circuit, sulphur deposition is possible in the formation and/or wellbore. For instance, the quantities of sulphur which could potentially separate in a 100,000 m.sup.3 / day sour gas producer at an isothermal pressure draw-down from 408 to 375 bar--which could for the greater part occur in the producing formation - may cause the separation of some 1100 kg/day of sulphur.
One of the worst consequences of sulphur deposition is that which takes place in the producing reservoir. Not only can this reduce production, but in extreme cases it can permanently shut off flow into the wellbore, leading to abandonment and the drilling of a replacement well. Formation plugging of this kind becomes more serious as the rock permeability becomes lower. Under these conditions, even the deposition of liquid sulphur in the pores can significantly reduce productivity since the viscosity of the liquid sulphur is much higher than that of the sour gas dense fluid phase.
To produce sour natural gas at a commercial rate of say 100,000 m.sup.3 / day via a conventional vertical production well the velocity of the gas through the pores of the reservoir formation at the proximity of the wellbore is inherently high. Due to the high gas velocity required to produce at commercial rates, the reservoir pressure in the proximity of the wellbore easily drops below the sulphur saturation pressure, creating conditions favorable for separation of elemental sulphur.
In field operations preventive and remedial methods have been developed and routinely used to cope with the problem of sulphur deposition in well tubulars. However, no practical, effective methods exist which prevent or remove sulphur deposits formed in the reservoir.